Oxygen-centred Radicals

Transformations of the radical cations of 2,3- and 2,5-dihydrofuran (DHF), radi-olytically generated in Freon matrices, were investigated by low-temperature EPR. The 2,3-DHF+ radical cation is stable at 77 K but at higher temperatures is transformed into dihydrofuryl radical, DHF. The oxygen-centred radical cation 2,5-DHF+ is unstable at 77 K and transforms via an intramolecular H-shift into a transient distonic radical cation 2,4-DHF+ which at higher temperatures yields the DHF radical. 

Studies examining H-abstraction by various oxygen-centred radicals have been reported. Activation energies114 and rate constants115 for the processes of abstraction from alkyl, vinyl, and aryl hydrocarbons have been calculated with abstraction from aliphatic C—H bonds proceeding with the highest activation energies. 

In trapping experiments, nitroxides will only trap carbon-centred radicals, and not oxygen-centred ones. This is particularly important since oxygen-centred radicals are often used as initiators (Section 10.2). The nitroxide should also not undergo other reactions, such as addition to double bonds or H-abstraction this increases the probability that it will trap selectively carbon-centred radicals which act as chain carriers in many synthetically useful organic reactions, as propagating species in polymerisations and as reactive intermediates in biological pathways. 

The oxidative coupling uses a copper-catalysed system and a base, usually an aliphatic or heterocyclic amine, and oxygen as the oxidizing agent. In broad terms, free-radical processes are involved to explain the polymerization pathway which involves formation of the phenoxide radical, and coupling of two radicals through the attack by an oxygen-centred radical at the para position of another phenolic molecule (Scheme 25). 

If a stabilizer is present then this reaction sequence is modified (Figure 2) by the competing reactions for the removal of polymer alkyl (P-) and alkyl peroxy (POO) radicals. The nitroxide, R2NO, whether pre-formed, as in the case of TMDBIO above, or formed in situ through the oxidation of a hindered amine stabilizer, may scavenge only carbon centred radicals, P-. The hindered phenol antioxidant, AH, in contrast, will scavenge only oxygen centred radicals, and in... 

The effect of ozone on saturated polymers, polyolefins (PO) and polystyrene (PS) in particular, is underrated. Ozone is involved in the initiation phase of their oxidation. Its attack results in formation of R, RO, ROO, ROOH and I CO species in PO, and free radicals and polymeric peroxides in PS. Moreover, ozone decomposes hydroperoxides into oxygen-centred radicals RO, ROO and HO. ... 

The first is the compilation of Denisov [389] which contains a chapter on the substitution reactions of atoms and radicals listing about 2000 rate coefficients. The reactions are classified as follows (i) reactions of atoms, (ii) reactions of oxygen centred radicals, (iii) reactions of carbon centred... 

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